1. Field of the Invention
The present invention relates to an apparatus mounted on a vehicle for assisting a vehicle driver in carefully driving.
2. Description of the Related Art
There have so far been proposed a wide variety of navigation apparatus which inform the vehicle driver of not only the current location of the vehicle but also an optimum travel route which extends from the current location of the vehicle to a destination specified by the vehicle driver. In order to estimate the current location of the vehicle, the navigation apparatus employs a so-called "dead reckoning method" in which the location of the vehicle is detected by heading and distance sensors mounted on the vehicle. The heading sensor is adapted to detect a heading of the vehicle, while the distance sensor is adapted to detect a travel distance of the vehicle. The dead reckoning method is in no need of external assisting facilities such as beacons and satellites. However, the heading and distance sensors entail errors in response to increase of the travel distance of the vehicle, thereby causing the errors of the location detection to be accumulated.
In order to reduce such accumulated errors of the location detection, the navigation apparatus employs a so-called "map matching algorithm" in which the current location of the vehicle is estimated by matching the patterns between a pattern of the travel route derived from the heading and distance sensors and a road pattern derived from a road map data base. Recently, the navigation apparatus further employs a "global positioning system" (hereinlater simply referred to as "GPS") for detecting the absolute position of the vehicle so as to correct the vehicle location derived by the foregoing map matching algorithm on the basis of the absolute position derived from the GPS, thereby enhancing the accuracy of the map matching algorithm. The GPS is a system for detecting an absolute position on the basis of radio waves transmitted from man-made satellites (hereinlater simply referred to as "GPS satellites"). More specifically, the radio waves are transmitted from three or more GPS satellites revolving around their predetermined orbits to a GPS receiver mounted on the vehicle. The GPS receiver is designed to decode pseudo-noise codes received from the three or four GPS satellites for calculating a two-dimensional or three-dimensional absolute location of the vehicle on the earth.
The navigation apparatus thus constructed is operated to display a road map including the current location of the vehicle and the destination and guide the vehicle driver to the destination. If an operator such as a vehicle driver or a passenger operates the navigation apparatus and designates a starting point, a transit point, and a destination, the navigation apparatus calculates a plurality of candidate routes between the starting point and the destination, and a plurality of indicators of a route selection. The indicators comprise, for example, a distance between the starting point and the destination, and a time required in a travel between the starting point and the destination. The navigation apparatus selects and displays the most recommendable one of the candidate routes with reference to the indicators. If -the vehicle approaches an intersection, a navigation apparatus recently proposed informs the vehicle driver of a road included in the most recommendable route, i.e., instructs the vehicle driver to turn, for instance, to the left.
There has also been proposed a wide variety of anomalous travel preventing apparatus which prevent the travel of the vehicle from becoming anomalous. The anomalous travel of the vehicle results, for example, from a drowsiness which comes over the vehicle driver. In this case, the anomalous travel preventing apparatus serves as a drowsiness preventing apparatus. When the drowsiness preventing apparatus detects one of indications that a travel locus of the vehicle is anomalous and that the vehicle extremely approaches an obstruction such as a preceding vehicle, the drowsiness preventing apparatus recognizes that a drowsiness comes over the vehicle driver. The former drowsiness detecting method is disclosed, for instance, in Japanese Patent Laid-open Publication Nos. 60-157927, 1-83423 and 5-69757. The drowsiness detecting method disclosed in the Japanese Patent Laid-open Publication No. 60-157927 comprises steps of obtaining a rotation angle signal representative of a rotation angle of a steering, analyzing the rotational signal, and recognizing the travel of the vehicle to be anomalous when high-frequency components are detected at the analyzing step. The drowsiness detection method disclosed in the Japanese Patent Laid-open Publication No. 1-83423 comprises steps of detecting lines drawn on a road so as to defining traffic lanes, and recognizing that the vehicle is moving in a zigzag direction when the vehicle frequency crosses the lines. The drowsiness detection method disclosed in the Japanese Patent Laid-open Publication No. 5-69757 comprises steps of detecting sideward deflection of the vehicle with respect to a position suitable for travel on a traffic lane, and recognizing the anomalous travel of the vehicle when the sideward deflection exceeds a predetermined deflection level.
If both the former prior-art navigation apparatus and the latter prior-art anomalous travel preventing apparatus are mounted on a vehicle, a drawback is encountered in that the vehicle is provided with numerous sensors, calculating devices, and informing devices for guiding the vehicle driver to the destination and preventing the anomalous travel of the vehicle. Another drawback is encountered in that the vehicle driver is confused by a lot of disorderly information and instructions provided for the vehicle driver because of the fact that the navigation apparatus and the anomalous travel preventing apparatus are operated independently of each other.
If only the latter prior-art anomalous travel preventing apparatus is mounted on a vehicle, a drawback is encountered in that mistaken information and instructions is frequently provided for the vehicle driver for the following reason. The prior-art anomalous travel preventing apparatus recognize the travel of the vehicle to be anomalous whenever the anomalous travel preventing apparatus detects one of indications of the anomalous travel of the vehicle. If there is a road improvement in a road on which the vehicle is traveling, the road improvement might urge the vehicle to travel in a zigzag direction or to frequently crosses lines drawn on the road. In this instance, there is a strong possibility that the anomalous travel preventing apparatus mistakenly recognizes the travel of the vehicle to be anomalous and accordingly gives an unnecessary caution to the vehicle driver by an alarm such as beep. In addition, the anomalous travel preventing apparatus mistakenly recognizes the travel of the vehicle to be anomalous if the vehicle meets traffic congestion. This can be understood because of the fact that the vehicle frequently approach the preceding vehicle in the extreme under the traffic congestion. If the vehicle driver feels the alarm to be useless and offensive to the ear, the anomalous travel preventing apparatus might be halted by the vehicle driver.